This invention relates to a method for monitoring the state of a lead acid battery such as the residual capacity, the residual lifetime or the like by measuring an internal impedance of the lead acid battery.
Hitherto, a lead acid battery has been used as a power supply for use in emergency in some cases. The lead acid battery used for such a purpose is called a stationary lead acid battery. It is connected in parallel with a commercial power supply to a load and usually is charged by a small current from the commercial power supply to maintain the capacity of the lead acid storage in 100% state, which is called a float charging. When an irregular condition such as a power outage occurs in the commercial power supply, electric power is supplied to the load from the lead acid battery instead of the commercial power supply.
As such a stationary lead acid battery, a large number of lead acid batteries are connected in series for use. Since such a lead acid battery is subjected to the float charging all the time, it is known that the lead acid battery is gradually degraded and leads to a state in which it cannot supply sufficient power to the load in the end. Thus, to keep track of whether or not sufficient power can be supplied when the commercial power supply falls into an irregular condition, namely, whether or not the capacity of the lead acid battery (residual capacity) is a predetermined amount, the following methods are adopted:    (1) monitoring change in the battery voltage of the lead acid battery and estimating the residual capacity based on the change;    (2) discharging the lead acid battery and estimating the residual capacity;    (3) quickly discharging the lead acid battery for a short time and estimating the residual capacity from the lowering degree of the voltage; and    (4) estimating the residual capacity of the lead acid storage from the value of internal resistance because the internal resistance rises gradually as the lead acid storage is degraded.
According to any of these methods, the state of the lead acid storage is monitored and when the residual capacity reaches a predetermined value or less, the lead acid battery is determined to be at the end of its lifetime and is replaced with a new one.
However, the above methods involve the following problems.
According to the method (1), there are voltage variations caused by the battery proper difference and to connect a large number of lead acid batteries in series for use, the variations are added and thus become large and the method lacks precision and a sufficient margin must be provided for monitoring on practical use.
The method (2) is precise, but a unit for discharging must be installed and when an anomaly occurs in the commercial power supply during the discharging time period, it becomes impossible to deal with the anomaly.
In the method (3), a discharging unit must also be installed. The method (4) is suitable because the error in the measured results is relatively small and does not require a discharging unit, but long-term use of the lead acid battery is desired and more precise state monitoring is desired.
So-called AC four-terminal method is known as a measurement method of the internal impedance of respective lead acid batteries connected in series as a lead acid battery unit.
The AC four-terminal method is a technique of making an AC current flow into the lead acid battery whose internal impedance is to be measured and measuring the electromotive force occurring at the time, thereby finding the internal impedance of the lead acid battery.
FIG. 13 shows a principle of the internal impedance measurement of a lead acid battery by the AC four-terminal method. In this figure, numeral 11 denotes a lead acid battery, numeral 12 denotes a lead acid battery unit, numeral 13 denotes an AC current supplier, and numeral 14 denotes an AC voltage instrument. The AC current supplier 13 and the AC voltage instrument 14 are connected in parallel with the lead acid battery 11.
The lead acid battery unit 12 comprises a plurality of lead acid batteries 11 connected in series to provide the objective voltage value. For example, the electromotive force of each lead acid battery is about 2 V. Six such lead acid batteries are connected in series to provide an electromotive force of about 12 V, which is defined as the lead acid battery unit 12 in the specification.
The AC current supplier 13 supplies an AC current to measure the internal impedance of the lead acid battery 11 (referred to as measurement current in the specification). An AC constant current source serves as the AC current supplier 13, for example, and the internal impedance thereof is infinite on principle.
The AC voltage instrument 14 measures the electromotive force occurring in the lead acid battery 11 by the current supplied by the AC current supplier 13. An AC voltmeter serves as The AC voltage instrument 14, for example, and the internal impedance thereof is infinite on principle.
It is known that the internal impedance of a lead acid battery changes depending on the temperature of the lead acid battery itself and the operating environment temperature. To solve this problem, the following documents disclose that the measurement result of the internal impedance is corrected based on the temperature of the lead acid battery itself.                Japanese Patent Publication No. 6-194428 (cf. column 9, line 29-column 11, line 1; FIGS. 7-9); and        Japanese Patent Publication No. 11-7985 (cf. column 1, line 32-column 2, line 25; FIGS. 1 and 2)        
The above documents indicate the concept for correcting the measurement result of the internal impedance based on the temperature of the lead acid battery itself; however, the temperature dependency is also varied in accordance with the internal impedance variation in each lead acid battery. In order to solve the problem, it is insufficient to make only a temperature correction based on a temperature correction as taught by the above documents.